Steam turbines typically comprise static nozzle segments that direct the flow of steam onto rotating turbine blades or buckets that are connected to a rotor. In steam turbines, the nozzle, which may form an airfoil or blade, is typically called a diaphragm stage.
In general, diaphragm stages are constructed using one of two methods. A first method uses a band/ring construction wherein the airfoils are first welded between inner and outer bands, which extend about 180°. Those arcuate bands with welded airfoils are then assembled and welded between the inner and outer carrier rings of the stator of the turbine. The second construction method consists of having the airfoils or blades of the nozzle welded directly to inner and outer rings. In this method, the nozzles generally have integral sidewalls that are used to make the interface with the inner and outer rings. This method is typically used for larger steam turbine units where access for creating the weld is available.
There are inherent limitations using the band/ring method of construction. A principle limitation in the band/ring assembly method is the distortion that occurs to the flowpath because of the weld that is used. That is, the weld used for these assemblies is of considerable size and heat input. The weld either requires high heat input and a significant quantity of metal filler or is very deep electron beam welds. In either case, the material or heat input causes the flow path to significantly distort. For example, material shrinkage causes the airfoils to bow outward from their designed shaped into the flow path. In many cases, the airfoils of the nozzle assemblies require adjustment and stress relief after welding.
The result of the steam path distortion (which may be present in some degree even after corrective post-assembly measures are taken) is reduced diaphragm stage efficiency. The surface profiles of the inner and outer bands also may change as a result of welding the nozzles into the stator assembly further causing an irregular flow path. More specifically, the nozzles and bands generally bend and distort as a result of conventional installation methods. This requires substantial finishing of the nozzle configuration to bring it into design specifications. In many cases, approximately 30% of the costs of the overall construction of the nozzle assembly is spent on deforming the nozzle assembly, including after welding and stress relief, to bring it back to its design configuration.
The second nozzle construction method (i.e., having the sidewalls of the airfoils or blades of the nozzle welded directly to the inner and outer rings) also has significant issues and inefficiencies. For example, conventional assembly methods that use a single nozzle construction welded into rings lack the proper configuration to promote a determined weld depth at the interface, which generally causes problems to arise. Further, conventional systems lack assembly alignment features on both the inner and outer ring, which may aid in installation. Also, conventional systems lack retainment features that may hold the installed nozzle in place in the event of a weld failure. Finally, conventional systems require time-consuming welds at both of the nozzle-inner ring interface and the nozzle-outer ring interface.
In addition, in the first stage of a double flow steam turbine, many of the issues associated with the construction of the nozzle assemblies may be exacerbated. However, certain characteristics of the first stage, which is often referred to as the tub stage, offer design opportunities that may be used to simplify nozzle assembly in that stage and make the assembly process more efficient. For example, the flow-splitter takes the place of the inner ring in the first stage and has beneficial characteristics that may be used. As discussed in more detail below, conventional nozzle design has failed to take advantage of these opportunities.
Accordingly, there is a need for a first stage nozzle that is designed to be installed by either sliding the nozzle into place or with limited low input heat welds or both. In either case, such assembly will minimize or eliminate steam path distortion that results from conventional welding processes, as well as improving production and cycle costs by making assembly more efficient. Further, there is a need for a first stage nozzle assembly that facilitates alignment of nozzle assembly during installation and creates a mechanical lock to prevent downstream movement of the nozzle assembly in the event of a weld failure. Certain unique characteristics of the first stage, which are not found in the downstream stages, may be taken advantage of in first stage nozzle design to efficiently satisfy these demonstrated needs.